I first heard of Joe Polchinski in 1988, when I was applying to various graduate schools in physics. During a visit to Harvard, I talked with Sidney Coleman, one of the leading thinkers in the esoteric world of quantum field theory. Although he was happy to sing the praises of his own institution, Sidney couldn’t help but add, “But I wouldn’t blame you if you went to the University of Texas. Whoever gets Joe Polchinski as an advisor will be fortunate indeed.”

Joe Polchinski. Credit: Lubos Motl Flickr 

I didn’t follow the advice, but I remembered the name, and a few years later I had the wonderful good fortune of being a postdoctoral researcher at the Kavli Institute for Theoretical Physics in Santa Barbara. Joe had moved there in 1992, and my office was just down the hall from his. I can’t tell you how often I would knock on his door to ask a question about physics. In a milieu packed with very smart people, he was the go-to guy, renowned for his carefulness and open-mindedness to new ideas. It wasn’t just me, either; countless students and colleagues sought out just a bit of his time. Eventually, as he worked to finish his giant two-volume textbook on string theory, he took to simply closing his door and pretending he wasn’t in the office. Once the book was finished, however, the door was open again, and the constant stream of visitors resumed.

Joe died at his home last week, age 63, after having been treated for brain cancer for a few years. His passing leaves a hole in the physics community, as his research was as innovative and impactful as ever. Looking over the countless memories and sympathies posted online, I don’t think I’ve ever seen such a large and heartfelt outpouring of grief at the passing of a great physicist.

Last year, as his illness made it increasingly hard to focus on the rigors of theoretical physics, Joe took the opportunity to write a lengthy scientific memoir. Even in the account of his childhood and college years—where his self-description included “shyness and lack of common sense”—we can discern the qualities that would later make him a uniquely insightful physicist. He was never one to chase the latest fad, or to follow the usual obvious strategies to become a well-known scientist—he didn’t write a huge number of papers, or vigorously market his own research. On the contrary, he seemed happiest when he could think deeply about some well-worn problem that had already been picked over by countless people before him. More often than not, he would return with some insight that would change how the rest of us thought about the subject.

Perhaps the single biggest idea Joe contributed in this way was the importance of “D-branes” in string theory. The basic idea of string theory is simply to replace the point particles (electrons, quarks, etc.) of ordinary quantum field theory by tiny one-dimensional loops or pieces of string. Given the leap from zero-dimensional particles to one-dimensional strings, it’s natural to ask whether we should also think about two-dimensional membranes, or even other higher-dimensional “branes.” (String theorists believe that space has more than the there dimensions we see around us, so they have no trouble thinking about, for example, a “five-brane” with extent in five spatial directions.) Generally the answer was thought to be “no, there’s no real benefit and certainly no requirement that such things exist.”

But in 1995 Joe realized that the opposite was true: properly considered, string theory is not just a theory of strings, but also includes various other extended objects, which for technical reasons are called D-branes. This idea helped launch the Second Superstring Revolution (after the first one in the early 1980’s), and led to the realization that all of the known varieties of string theory were simply different manifestations of one underlying theory.

The discovery of the importance of D-branes led to another world-changing idea: the string theory landscape. Those extra dimensions that string theory predicts have to be hidden somehow, and the simplest thing to do is to curl them up into an invisibly tiny space. It turns out that D-branes can wrap around different directions in those compact spaces, and Joe and Raphael Bousso argued that this could lead to a preposterously large number of different “vacua” predicted by string theory. Each vacuum would manifest different low-energy laws of physics, such as the number of particles and the forces between them.

Coupled with inflationary cosmology, we might even imagine an immense multiverse of many regions, each of which is found in a different vacuum. Then, plausibly, we could try to explain certain features of the vacuum in which we find ourselves by appealing to the anthropic principle—most such vacua would be inhospitable to the existence of intelligent life, so the one in which we live would need to have special properties.

Joe never really liked such anthropic reasoning, but he was led to consider it by following the equations, and like a good scientist he accepted what he found once he was there. But when I was working at KITP he was still a confirmed skeptic. I once asked him what his favorite explanation would be if astronomers were to discovery a nonzero cosmological constant (the energy of empty space itself). In his always-quotable way, Joe immediately replied that this would be a disaster, as the only plausible answer would be the anthropic principle. That would be sad, as it would represent a failure of physics to make a unique prediction for a physically important quantity; he would probably have to quite doing physics under such conditions, he mused, and if that happened, he promised that I could have his office.

In 1998, of course, astronomers did indeed discover that the universe is accelerating, a sign of a nonzero cosmological constant. I reminded Joe of his promise, but instead of retiring he decided to continue doing interesting physics, and he kept his office. That’s okay, he put it to better use than I would have.

Most recently, Joe and collaborators proposed the “firewall paradox” for black holes. If we want to reconcile locality (physical interactions only happen between nearby points in space) with the conservation of quantum information as black holes evaporate over time, they argued that observers falling into such black holes would be irradiated by ultra-high-energy particles, contrary to everything you would be taught in a general relativity textbook. The thought experiment was absolutely typical Joe: carefully thinking about a problem that had been around for years, reaching an astonishing and hard-to-believe conclusion, and then remaining agnostic about how the apparent paradox would ultimately be resolved.

Joe Polchinski was a theoretical physicist’s theoretical physicist. The world is a little less insightful without him around.

 

Sean Carroll is a theoretical physicist at the California Institute of Technology in Pasadena, and author of The Big Picture: On the Origins of Life, Meaning, and the Universe Itself.